Patterned magnetic recording media with discrete magnetic regions separated by regions of antiferromagnetically coupled films

ABSTRACT

A magnetic recording disk is patterned into discrete magnetic and nonmagnetic regions with the magnetic regions serving as the magnetic recording data bits. The magnetic recording layer comprises two ferromagnetic films separated by a nonferromagnetic spacer film. The spacer film material composition and thickness is selected such that the first and second ferromagnetic films are antiferromagnetically coupled across the spacer film. After this magnetic recording layer has been formed on the disk substrate, ions are irradiated onto it through a patterned mask. The ions disrupt the spacer film and thereby destroy the antiferromagnetic coupling between the two ferromagnetic films. As a result, in the regions of the magnetic recording layer that are ion-irradiated the first and second ferromagnetic films are essentially ferromagnetically coupled so that the magnetic moments from the ferromagnetic films are parallel and produce a magnetic moment that is essentially the sum of the moments from the two films. In the non-irradiated regions of the magnetic recording layer, the first and second ferromagnetic films remain antiferromagnetically coupled so that their magnetic moments are oriented antiparallel. The composition and thicknesses of the first and second ferromagnetic films are selected such that essentially no magnetic field is detectable at a predetermined distance above the magnetic recording layer corresponding to the height that the magnetic recording head would be located.

TECHNICAL FIELD

This invention relates generally to magnetic recording media, and moreparticularly to patterned magnetic recording disks with discretemagnetic regions.

BACKGROUND OF THE INVENTION

Conventional magnetic recording media, such as the magnetic recordingdisks in hard disk drives, typically use a granular ferromagnetic layer,such as a sputter-deposited cobalt-platinum (CoPt) alloy, as therecording medium. Each magnetized domain in the magnetic layer iscomprised of many small magnetic grains. The transitions betweenmagnetized domains represent the “bits” of the recorded data. IBM's U.S.Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventionalrigid magnetic recording disk.

The challenge of producing continuous granular films as magnetic mediawill grow with the trend toward higher areal storage densities. Reducingthe size of the magnetic bits while maintaining a satisfactorysignal-to-noise ratio, for example, requires decreasing the size of thegrains. Unfortunately, significantly reducing the size of weaklymagnetically coupled magnetic grains will make their magnetizationunstable at normal operating temperature. To postpone the arrival ofthis fundamental “superparamagnetic” limit and to avert otherdifficulties associated with extending continuous granular media, therehas been renewed interest in patterned magnetic media.

With patterned media, the continuous granular magnetic film that coversthe disk substrate is replaced by an array of spatially separateddiscrete magnetic regions or islands, each of which serves as a singlemagnetic bit. The primary approach for producing patterned media hasbeen the use of lithographic processes to selectively deposit or removemagnetic material from a magnetic layer on the substrate so thatmagnetic regions are isolated from one another and surrounded by areasof nonmagnetic material. Examples of patterned magnetic media made withthese types of lithographic processes are described in U.S. Pat. Nos.5,587,223; 5,768,075 and 5,820,769.

From a manufacturing perspective, an undesirable aspect of the processfor patterning media that requires the deposition or removal of materialis that it requires potentially disruptive processing with the magneticmedia in place. Processes required for the effective removal of resistsand for the reliable lift-off of fine metal features over large areascan damage the material left behind and therefore lower productionyields. Also, these processes must leave a surface that is clean enoughso that the magnetic read/write head supported on the air-bearing sliderof the disk drive can fly over the disk surface at very low flyingheights, typically below 30 nanometers (nm).

An ion-irradiation patterning technique that avoids the selectivedeposition or removal of magnetic material, but uses a special type ofperpendicular magnetic recording media, is described by Chappert et al,in “Planar patterned magnetic media obtained by ion irradiation”,Science, Vol. 280, Jun. 19, 1998, pp. 1919-1922. In this techniquePt—Co—Pt multilayer sandwiches which exhibit perpendicularmagnetocrystalline anisotropy are irradiated with ions through alithographically patterned mask. The ions mix the Co and Pt atoms at thelayer interfaces and reorient the easy axis of magnetization to bein-plane so that the irradiated regions no longer have perpendicularmagnetocrytalline anisotropy.

IBM's application Ser. No. 09/350,803, filed Jul. 9, 1999, now U.S. Pat.No. 6,331,364, describes an ion-irradiated patterned disk that uses acontinuous magnetic film of a chemically-ordered Co (or Fe) and Pt (orPd) alloy with a tetragonal crystalline structure. The ions causedisordering in the film and produce regions in the film that are lowcoercivity or magnetically “soft” and have no magnetocrystallineanisotropy.

A potential disadvantage of the Chappert et al. and IBM ion-irradiatedpatterned disks is that the regions separating the discrete magneticregions from one another are not completely nonmagnetic, but sill havesome magnetic properties. Thus the magnetoresistive read head in thedisk drive will detect noise and/or some type of signal from theseregions. In addition, these ion irradiation techniques require the useof a mask that is difficult to fabricate because the holes in the maskare used to generate corresponding nonmagnetic regions on the disk,whereas it is desirable to use a mask that has the same hole pattern asthe resulting magnetic bits on the disk.

What is needed is a patterned magnetic recording disk that has discretemagnetic regions separated by completely nonmagnetic regions so thatonly the magnetic regions contribute to the read signal, and that ismade by a patterning technique where the mask pattern of holes matchesthe pattern of discrete magnetic regions of the disk.

SUMMARY OF THE INVENTION

The present invention is a magnetic recording disk that is patternedinto discrete magnetic and nonmagnetic regions with the magnetic regionsserving as the magnetic recording data bits. The magnetic recordinglayer comprises two ferromagnetic films separated by a nonferromagneticspacer film. The spacer film material composition and thickness isselected such that the first and second ferromagnetic films areantiferromagnetically coupled across the spacer film. After thismagnetic recording layer has been formed on the disk substrate, ions areirradiated onto it through a patterned mask. The ions disrupt the spacerfilm and thereby destroy the antiferromagnetic coupling between the twoferromagnetic films. As a result, in the regions of the magneticrecording layer that are ion-irradiated the first and secondferromagnetic films are essentially ferromagnetically coupled so thatthe magnetic moments from the ferromagnetic films are parallel andproduce a magnetic moment that is essentially the sum of the momentsfrom the two films. In the non-irradiated regions of the magneticrecording layer, the first and second ferromagnetic films remainantiferromagnetically coupled so that their magnetic moments areoriented antiparallel. The composition and thicknesses of the first andsecond ferromagnetic films are selected such that essentially nomagnetic field is detectable at a predetermined distance above themagnetic recording layer corresponding to the height that the magneticrecording head would be located

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of the magnetic recording diskshowing the antiferromagnetically (AF) coupled layer prior topatterning.

FIG. 2 is a schematic illustration of the process for patterning theAF-coupled layer by ion irradiation through a stencil mask.

FIG. 3 is a magnetic force microscopy (MFM) image of the patternedAF-coupled layer showing discrete oblong-shaped magnetic regions.

FIGS. 4A-4B are schematic views of the disk structure of the presentinvention showing different magnetic bit states with their correspondingsignal profiles.

DETAILED DESCRIPTION OF THE INVENTION

Preferred Embodiments

The magnetic recording medium of the present invention is made by firstforming a continuous (non-patterned) magnetic layer of two or moreferromagnetic films that are exchange-coupled antiferromagnetically (AF)to their neighboring ferromagnetic films by one or more nonferromagneticspacer films. FIG. 1 illustrates the cross sectional layer structure ofthe disk 10 with the AF-coupled magnetic layer 20 prior to patterning.

The disk substrate 11 may be made or any suitable material, such sglass, SiC/Si, ceramic, quartz, or an AlMg alloy base with a NiP surfacecoating. The seed layer 12 is an optional layer that may be used toimprove the growth of the underlayer 13. The seed layer 12 is mostcommonly used when the substrate 11 is nonmetallic, such as glass. Theseed layer 12 has a thickness in the range of approximately 0.5 to 5 nmand is one of the materials, such as Ta, CrTi or NiAl, which are knownin the prior art to be useful as seed materials for promoting the growthof subsequently deposited layers in certain preferred crystallineorientations. The underlayer 13 is deposited onto the seed layer, ifpresent, or otherwise directly onto the substrate 11, and is anonmagnetic material such as chromium or a chromium alloy, such as CrVor CrTi. The underlayer 13 has a thickness in the range of 5 to 1000 nmwith a typical value being approximately 50 nm.

The AF-coupled magnetic layer 20 is made up of two ferromagnetic films22, 24 separated by a nonferromagnetic spacer film 26. Thenonferromagnetic spacer film 26 thickness and composition are chosen sothat the magnetic moments 32, 34 of adjacent films 22, 24, respectively,are AF-coupled through the nonferromagnetic spacer film 26 and areantiparallel in zero applied field. In the preferred embodiment the twoAF-coupled films 22, 24 of layer 20 have magnetic moments that areoriented antiparallel with the lower film 24 having a larger moment sothat the net magnetic field from the AF-coupled layer 20 is close tozero at the recording head located above the disk. Each of theferromagnetic films 22, 24 is preferably a Co alloy, such as a CoPtCrBalloy with 4 to 20 atomic percent (at. %) platinum, 10 to 23 at. %chromium and 2 to 20 at. % boron, and the nonferromagnetic spacer film26 is preferably ruthenium (Ru).

Before the deposition of the first ferromagnetic film 24 of theAF-coupled magnetic layer 20, a very thin (typically 1 to 5 nm) Co alloyonset or nucleation layer 14 is typically deposited on the underlayer13. The nucleation layer 14 has a composition selected to enhance thegrowth of the hexagonal close-packed (HCP) Co alloy of film 24 so thatits C-axis is oriented in the plane of the layer. The nucleation layer14 may be a CoCr alloy with a Cr composition selected so as to make thelayer 14 nonferromagnetic or very slightly ferromagnetic. Alternatively,the nucleation layer 14 may be a ferromagnetic Co alloy, in which casethe nucleation layer 14 will affect the magnetic properties of the film24. If the film 24 is CoPtCrB, then the nucleation layer 14 may beCoPtCr or CoPtCrB with B less than 6 at. %. All of the layers describedabove from the seed layer 12 to the top ferromagnetic film 22 can besputtered in a continuous process in either an in-line sputtering systemor a single disk system, such as commercially available single disksystems with multiple sputtering target capacity. The sputter depositionof each of the layers can be accomplished using standard targets andtechniques known to those in the field with the modifications describedabove.

The AF coupling of ferromagnetic films via a nonferromagnetic transitionmetal spacer film, like the structure of layer 20 in FIG. 1, has beenextensively studied and described in the literature. In general, theexchange coupling oscillates from ferromagnetic to antiferromagneticwith increasing spacer film thickness. This oscillatory couplingrelationship for selected material combinations is described by Parkinet al. in “Oscillations in Exchange Coupling and Magnetoresistance inMetallic Superlattice Structures: Co/Ru, Co/Cr and Fe/Cr”, Phys. Rev.Lett. Vol. 64, p. 2034 (1990). The material combinations includeferromagnetic films made of Co, Fe, Ni, and their alloys, such as Ni—Fe,Ni—Co, and Fe—Co, and nonferromagnetic spacer films such as Ru, chromium(Cr), rhodium (Rh), iridium (Ir), copper (Cu), and their alloys. Foreach such material combination, the oscillatory exchange couplingrelationship has to be determined, if not already known, so that thethickness of the nonferromagnetic spacer films is selected to assureantiferromagnetic coupling between the two ferromagnetic films. Theperiod of oscillation depends on the nonferromagnetic spacer material,but the strength and phase of the oscillatory coupling also depends onthe ferromagnetic material and interfacial quality. The oscillatoryantiferromagnetic coupling of ferromagnetic films has been used inspin-valve type giant magnetoresistance (GMR) recording heads to designcontinuous magnetized antiferromagnetically coupled films whose magneticmoments are rigidly coupled together antiparallel during operation ofthe head. These type of spin-valve structures are described, forexample, in IBM U.S. Pat. Nos. 5,408,377 and 5,465,185. The '185 patentdescribes a structure used in many commercially available spin-valve GMRheads, namely a laminated antiparallel pinned ferromagnetic layer havingferromagnetic films whose moments are rigidly coupled together andremain stationary during operation of the head. This type of magneticstructure of two ferromagnetic films antiferromagnetically coupledacross a very thin nonferromagnetic spacer film, such as used inspin-valve heads and show in the AF-coupled magnetic layer 20 of FIG. 1,is also called a “synthetic antiferromagnetic”. In the case where thestructure has no net magnetic moment because the moments from theindividual ferromagnetic films cancel, the structure can be called a“compensated” synthetic antiferromagnet.

For this AF-coupled structure of layer 20 the orientations of themagnetic moments 32, 34 of adjacent films 22, 24, respectively, arealigned antiparallel and thus add destructively. The arrows 32, 34represent the moment orientations of individual magnetic domains thatare directly above and below one another across the AF coupling film 26.In the absence of an applied magnetic field, when the bottomferromagnetic film 24 is deposited onto the nucleating layer 14, it willhave a granular structure with multiple adjacent grains being coupledtogether to form individual magnetic domains. In the absence of anapplied magnetic field the moments of these domains in film 24 will beessentially randomly oriented. The spacer film or AF-coupling film 26 isthen deposited to the correct thickness directly on ferromagnetic film24. Next, the second or top ferromagnetic film 22 is deposited directlyon the AF coupling film 26. As the grains of ferromagnetic film 22 growthey will form magnetic domains with moment orientations that areantiparallel to the moment orientations of ferromagnetic film 24 thatare directly across the AF coupling film 26.

The type of ferromagnetic material and the thickness values t₁, t₂ ofthe ferromagnetic films 22, 24 are chosen so that the field strengthabove the disk at the height where the recording head is located isessentially equal for the two films. The Mrt for the layer 20 is givenby Mr₁t₁-Mr₂t₂. In the preferred embodiment, Mr₁t₁ should be less thanMr₂t₂ because film 22 is closer to the head. This may be accomplished byusing the same ferromagnetic materials in the two films 22, 24 andadjusting t₁ and t₂. If different ferromagnetic material compositionsare used in the two films 22, 24 so that the magnetization (the magneticmoment per unit volume of material) of the two ferromagnetic films ismade different, then the thicknesses are adjusted accordingly. In analterative embodiment, the two films 22, 24 can be with Mr₁t₁=Mr₂t₂ sothat the layer 20 has substantially zero net magnetic moment. In thatcase there would be some small field detected at the head because theupper film 22 is closer to the head.

While FIG. 1 is shown for a AF-coupled magnetic layer 20 with a two-filmstructure and a single spacer film, the invention is extendible tostructures with multiple spacer films and multiple ferromagnetic films.

The nonferromagnetic spacer film 26 in FIG. 1 is a 0.6 nm Ru film. TheRu spacer film thickness was chosen to be at the first antiferromagneticpeak in the oscillatory coupling relationship. It may also be desirablefor each of the CoPtCrB ferromagnetic films 22, 24 to include aninterface film consisting essentially of 0.5 nm of Co at the interfacewith the Ru spacer film 26. These ultra-thin Co films increase theinterfacial moment between the ferromagnetic films and the spacer film,resulting in enhanced antiferromagnetic coupling. However,antiferromagnetic exchange coupling will occur without incorporating theCo interface films in the CoPtCrB ferromagnetic films 22, 24.

After the AF-coupled magnetic layer 20 has been formed it is patternedto form discrete isolated magnetic regions separated from one another by“nonmagnetic” regions that produce essentially no magnetic field at thehead. The discrete magnetic regions are sized to function as discretemagnetic domains or bits. This patterning is performed without the needfor selective deposition or removal of the magnetic material. Regions ofthe AF-coupled layer 20 are transformed by ion irradiation from twoferromagnetic films AF-coupled across the Ru spacer film 26 to twoferromagnetic films that are ferromagnetically coupled so that theirmagnetic moments are parallel.

In the preferred patterning method, a stencil mask is irradiated with adose of nitrogen ions (N+) and the ions are selectively transmittedthrough the holes in the mask. The ions pass through the holes in themask and impact the AF-coupled layer 20 in selected regionscorresponding to the pattern of the holes in the mask. The ions disruptthe interface between the Ru spacer film 26 and the ferromagnetic films22, 24, and destroy the AF-coupling of the ferromagnetic films 22, 24.This occurs essentially without disturbing the magnetic properties ofthe ferromagnetic films 22, 24 and creates regions where the twoferromagnetic films are coupled together ferromagnetically with theirmagnetic moments oriented parallel. The regions not impacted by the N+ions remain AF-coupled and thus have essentially no net magnetic momentas measured at the head. As a result the magnetic regions are separatedfrom one another by “nonmagnetic” regions that have no magnetization.Thus the pattern of holes in the mask corresponds to the pattern ofmagnetic bit regions on the disk.

FIG. 2 illustrates the patterning process schematically. The layer 20remains AF-coupled across the spacer film (represented by cross-hatchingof film 26) in the regions 52, 54 not aligned with a hole 56 in thesilicon stencil mask 60. In the region 55 of layer 20 that is alignedwith a hole 56 in mask 60, disordering of the interface between the Ruspacer film and the ferromagnetic films 22, 24 has occurred (asrepresented by the dotted area of film 26), and the magnetic moments ofthe two films 22, 24 become ferromagnetically coupled.

The stencil mask 60 is a non-contact mask that comprises a wafer, suchas silicon, with holes etched through it. The ions, depicted by arrows62, are transmitted through the holes in the wafer. The silicon stencilmask was fabricated from a commercial silicon-on-insulator (SOI) waferwith a 10 μm-thick top side silicon layer, 0.5 μm of SOI oxide, and a500 μm-thick silicon carrier substrate. The stencil holes were firstpatterned by optical lithography and then transferred into the 10μm-thick Si layer of SF₆-based, high aspect ratio reactive ion etching(RIE) with the SOI oxide serving as a reliable etch stop. Windows werethen etched from the back side through the carrier substrate, using asimilar RIE process, and the remaining SOI oxide was removed with a wetHF etch. The resulting silicon membrane is approximately 10 μm thick andcovers an area of 1×1 mm. The holes in the membrane are nominally 1 μmin diameter, although somewhat irregular in shape, and are replicatedthroughout its area with a regular spacing of 1 to 10 μm. In making thepatterned media two such stencil masks can be aligned with their holesoverlapping to create holes with effective diameters in the range of 100nm. However, it is possible to fabricate a single stencil mask in thismanner, with much smaller holes in the sub-100 nm range, to producepatterned media with the desired areal density. A detailed descriptionof the use of stencil masks for ion-beam patterning is described by B.D. Terris et al., “Ion-beam patterning of magnetic films using stencilmasks”, Appl. Phys. Lett., Vol. 75, No. 3, Jul. 19, 1999, which isincorporated herein by reference. In the preferred embodiment the maskhas holes formed in a pattern to form a magnetic recording disk withconcentric circular tracks, with each track having discrete magneticregions spaced along it to serve as the individually recordable magneticbits.

While nitrogen ions were used, other ion species that may be usedinclude ions of He, Ar, Ne, Kr and Xe. The voltage and dosage of the ionirradiation required to achieve the desired disruption of the interfacebetween the Ru spacer film 26 and the ferromagnetic films 22, 24 can bedetermined experimentally.

The preferred method for patterning the media with ion irradiation is bya non-contact mask, such as the silicon stencil mask described above.However, it is also possible to use conventional lithography, where aphotoresist is formed on the AF-coupled layer and then patterned toexpose openings aligned with portions of the layer intended to becomethe magnetic bit regions that are separated or isolated from the“nonmagnetic” regions.

After the AF-coupled layer has been patterned, a conventional protectiveovercoat (not shown in FIG. 1) can be formed over it to complete thefabrication of the patterned magnetic disk. The protective overcoat maybe a typical overcoat of sputter deposited essentially amorphous carbonoptionally doped with hydrogen and/or nitrogen. The overcoat istypically less than 15 nm thick.

Experimental Results

An AF-coupled structure of CoPtCrB/Co/Ru/Co/CoPtCrB was prepared on a 50nm Cr metal substrate. The ferromagnetic films of CoPtCrB werCo₆₈Pt₁₂Cr₂₀ alloy doped with 5 atomic percent (at. %) B. The lowerCoPtCrB film was 10 nm thick and the upper CoPtCrB film was 5 nm. Thetwo ferromagnetic films were interleaved with a Co(0.5 nm)/Ru(0.6nm)/Co(0.5 nm) trilayer that coupled the CoPtCrB filmsantiferromagnetically so their moments were oriented antiparallel. Thethickness of the CoPtCrB films was chosen such that there would beessentially zero magnetic filed at a predetermined distance above themagnetic layer. This distance is the height above the disk where theread head is located (i.e., the nominal flying height of the read head).Since the upper film 22 (see FIG. 1) is closer to the read head, thefield from it will be higher than the field from the lower film 24. Thusthe thickness of lower film 24 is made thicker to compensate and makethe net field essentially zero at the head.

The magnetization of this structure was then measured with a Kerr looperover a range of external applied magnetic fields. A very high magneticfield (e.g., 8 kOe) sufficient to overcome the antiferromagneticcoupling of the two ferromagnetic films was first applied in thenegative direction, and the Kerr data showed that the ferromagneticfilms had their moments aligned parallel with the applied fielddirection. The field was then reduced and the Kerr data showed one ofthe ferromagnetic films switching magnetization direction, near thefield strength that would correspond to the antiferromagnetic couplingfield, so that the ferromagnetic films then had their moments alignedantiparallel. As the field passed through zero toward a positive appliedfield the ferromagnetic film moments remained antiparallel until thepositive field exceeded the antiferromagnetic coupling field, at whichpoint the ferromagnetic film moments became oriented parallel to oneanother and aligned with the positive applied field direction. Thus theKerr data showed that this film structure is a syntheticantiferromagnet.

Next, this structure was bombarded with N+ ions at a dose of 2×10¹⁶ions/cm² at 700 ke V energy. When the structure was once again exposedto the same range of external applied field the Kerr data showed noAF-coupling of the ferromagnetic films. Instead the structure behavedlike a single ferromagnetic layer, indicating that the ion bombardmenthad destroyed the antiferromagnetic coupling across the Ru spacer film.One can thus conclude that the ion bombardment had disrupted theinterface between the Ru spacer film and the ferromagnetic films andintermixed the Ru with the adjacent ferromagnetic films. The structurehad full remanence and a coercivity of about 1500 Oe.

Patterning of this same type of AF-coupled structure was thendemonstrated using N+ ions. A 10 μm×10 μm area of this structure wasexposed to a dose of 6×10¹⁵ N+ ions/cm² through a Si stencil mask withmicron size oblong-shaped holes. After patterning, the structure wasfirst magnetized with a large magnetic field (20 kOe) in one direction.This field strength is sufficient to align the magnetization of thenon-irradiated regions and to overcome the AF-coupling field in thenon-irradiated regions so that the magnetizations of the ferromagneticfilms in the non-irritated regions are aligned parallel to one anotherand to the applied field. This field was then removed, which caused thetwo ferromagnetic films in the non-irradiated regions to becomeAF-coupled. Next, a field of 2 kOe was applied in the oppositedirection. This 2 kOe field is less than the AF-coupling field of thenon-irradiated regions but large enough to switch the magnetizationdirection of the ferromagnetically coupled films in the irradiatedregions only. FIG. 3 is a magnetic force microscopy (MFM) image of thepatterned structure, with the oblong-shaped regions being the irradiatedregions where the Ru spacer film in the structure was disrupted so thatthe ferromagnetic films in these oblong-shaped regions areferromagnetically coupled. The light and dark contrast lines on the longedges of the oblong-shaped regions originate from magnetic transitionsbetween the top (or bottom) ferromagnetic film in the non-irradiatedAF-coupled regions and the ferromagnetically coupled films in theoblong-shaped irradiated regions.

The two bit states in the recording media according to the presentinvention, wherein the magnetic field strength above the disk at theheight where the recording head is located is essentially equal for thetwo films 24, 22, are depicted schematically in FIGS. 4A-4B. Themagnetic transition regions are designated 80, 82. In FIG. 4A, only thetransitions between the top film magnetic states, 70-72 and 72-74,contribute to the signal S1 because the bottom film magnetic states,71-73 and 73-75, do not have magnetic transitions in the regions 80, 82.In FIG. 4B only the transitions between the bottom film magnetic states,71-73 and 73-75, contribute to the signal S2 because the top filmmagnetic states, 70-72 and 72-74, do not have magnetic transitions inthe regions 80, 82. In FIG. 4A the magnetization of the topferromagnetic film 22 of the AF-coupled non-irritated regions 70, 74 areoppositely aligned with the magnetization in the ferromagneticirradiated region 72-73, leading to a typical magnetic field profile asindicated by S1. This represents one written state, a “1”. Conversely,the other written state, a “0”, is accomplished by applying a fieldlarger than the coercivity of the ferromagnetic regions 72-73 butsmaller than the AF-coupling field between the top and bottom films70-71 and 74-75 in the AF-coupled regions. In this way, only theferromagnetic region 72-73 switches its magnetization and alignsparallel to the top films 70, 74. An inverted signal S2 is the result.The signals S1 (FIG. 4A) and S2 (FIG. 4B) depicted above the transitionregions 80, 82 show that while the signs of the signals from thetransitions are different, the amplitudes are the same, regardless ofthe directions of the transitions. This is because each of theferromagnetic films 22, 24 is designed to have a magnetic moment so thatthe fields from the films, as detected at the head, are the same, eventhough film 22 is farther from the head.

If the alternative embodiment were used, where Mr₁t₁=Mr₂t₂ so that thelayer 20 has substantially zero net magnetic moment, then S1 and S2would have different amplitudes. This is because the two films 22, 24would then have the same magnetic moments, but film 24 is farther fromthe head. Thus the signal S2 from the transitions in lower film 24 wouldhave a smaller amplitude than the signal S1.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit, scope, and teaching of theinvention. Accordingly, the disclosed invention is to be consideredmerely as illustrative and limited in scope only as specified in theappended claims.

1. A magnetic recording medium comprising: a substrate; a magnetic layeron the substrate and comprising a first ferromagnetic film, a secondferromagnetic film, and a nonferromagnetic film between the first andsecond ferromagnetic films; and wherein the magnetic layer is has aplurality of concentric data tracks, each of the data tracks beingpatterned along the track into first regions with the first and secondferromagnetic films being antiferromagnetically coupled across thenonferromagnetic film, and second regions with the first and secondferromagnetic films being ferromagnetically coupled, the second regionsbeing magnetically recordable data bits.
 2. The medium of claim 1wherein the first ferromagnetic film in the first regions has athickness t1 and a magnetization M1, the second ferromagnetic film inthe first regions has a thickness t2 and a magnetization M2, and whereinthe magnetic moment per unit area (M2×t2) is greater than the magneticmoment per unit area (M1×t1), whereby the magnetic field from the firstregions is essentially zero at a predetermined distance above themagnetic layer.
 3. The medium of claim 2 wherein the first and secondferromagnetic films are formed of substantially the same material, andwherein t2 is greater than t1.
 4. The medium of claim 1 wherein thenonferromagnetic film is formed of a material selected from the groupconsisting of ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium (Ir),copper (Cu), and their alloys.
 5. The medium of claim 1 wherein thefirst and second ferromagnetic films are made of a material selectedfrom the group consisting of Co, Fe, Ni, and their alloys.
 6. The mediumof claim 1 A magnetic recording medium comprising: a substrate; amagnetic layer on the substrate and comprising a first ferromagneticfilm, a second ferromagnetic film, and a nonferromagnetic film betweenthe first and second ferromagnetic films; and wherein the magnetic layeris patterned into first regions with the first and second ferromagneticfilms being antiferromagnetically coupled across the nonferromagneticfilm, and second regions with the first and second ferromagnetic filmbeing ferromagnetically coupled; wherein the first ferromagnetic filmincludes an interface film consisting essentially of cobalt located atthe interface of the first ferromagnetic film and the nonferromagneticfilm.
 7. The medium of claim 1 A magnetic recording medium comprising: asubstrate; a magnetic layer on the substrate and comprising a firstferromagnetic film, a second ferromagnetic film, and a nonferromagneticfilm between the first and second ferromagnetic films; and wherein themagnetic layer is patterned into first regions with the first and secondferromagnetic films being antiferromagnetically coupled across thenonferromagnetic film, and second regions with the first and secondferromagnetic film being ferromagnetically coupled; where the secondferromagnetic film includes an interface film consisting essentially ofcobalt located at the interface of the second ferromagnetic film and thenonferromagnetic film.
 8. The medium of claim 1 further comprising anonferromagnetic underlayer located on the substrate between thesubstrate and the magnetic layer.
 9. The medium of claim 1 furthercomprising a protective overcoat formed over the magnetic layer.
 10. Amagnetic recording disk comprising: a substrate; a nonferromagneticunderlayer on the substrate; a magnetic recording layer on theunderlayer and having a plurality of concentric data tracks, therecording layer comprising a first cobalt alloy ferromagnetic film, anonferromagnetic spacer film of a material selected from the groupconsisting of ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium (Ir),copper (Cu), and their alloys formed on and in contact with the firstferromagnetic film, and a second cobalt alloy ferromagnetic film formedon and in contact with the spacer film, each of the data tracks on themagnetic recording layer being patterned along the track into firstregions wherein the spacer film has a thickness sufficient to induce thesecond ferromagnetic film to be exchange coupled antiferromagneticallyto the first ferromagnetic film across the spacer film and secondregions wherein the first and second ferromagnetic films are notantiferromagnetically coupled, whereby each of said second regionsproduce produces a magnetic field a predetermined distance above themagnetic layer that is greater than the magnetic field from each of saidfirst regions, the magnetic fields above the second regions representingthe bits along the data track; and a protective overcoat formed on themagnetic recording layer.
 11. The disk of claim 10 wherein the first andsecond ferromagnetic films of the magnetic recording layer are formed ofsubstantially the same material.
 12. The disk of claim 10 wherein thefirst and second ferromagnetic films of the magnetic recording layer aremade of a material selected from the group consisting of Co, Fe, Ni, andtheir alloys.
 13. The disk of claim 10 A magnetic recording diskcomprising: a substrate; a nonferromagnetic underlayer on the substrate;a magnetic recording layer on the underlayer comprising a first cobaltalloy ferromagnetic film, a nonferromagnetic spacer film of a materialselected from the group consisting of ruthenium (Ru), chromium (Cr),rhodium (Rh), iridium (Ir), copper (Cu) and their alloys formed on andin contact with the first ferromagnetic film, and a second cobalt alloyferromagnetic film formed on and in contact with the spacer film, themagnetic recording layer being patterned into first regions wherein thespacer film having a thickness sufficient to induce the secondferromagnetic film to be exchange coupled antiferromagnetically to thefirst ferromagnetic film across the spacer film and second regionswherein the first and second ferromagnetic film are notantiferromagnetically coupled, whereby said second regions produce amagnetic field a predetermined distance above the magnetic layer that isgreater than the magnetic field from said first regions; and aprotective overcoat formed on the magnetic recording layer; wherein thefirst ferromagnetic film of the recording layer includes an interfacefilm consisting essentially of cobalt located at the interface of thefirst ferromagnetic film and the spacer film.
 14. The disk of claim 10 Amagnetic recording disk comprising: a substrate; a nonferromagneticunderlayer on the substrate; a magnetic recording layer on theunderlayer comprising a first cobalt alloy ferromagnetic film, anonferromagnetic spacer film of a material selected from the groupconsisting of ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium (Ir),copper (Cu), and their alloys formed on and in contact with the firstferromagnetic film, and a second cobalt alloy ferromagnetic film formedon and in contact with the spacer film, the magnetic recording layerbeing patterned into first regions wherein the spacer film has athickness sufficient to induce the second ferromagnetic film to beexchange coupled antiferromagnetically to the first ferromagnetic filmacross the spacer film and second regions wherein the first and secondferromagnetic films are not antiferromagnetically coupled, whereby saidsecond regions produce a magnetic field a predetermined distance abovethe magnetic layer that is greater than the magnetic field from saidfirst regions; and a protective overcoat formed on the magneticrecording layer; wherein the second ferromagnetic film of the recordinglayer includes an interface film consisting essentially of cobaltlocated at the interface of the second ferromagnetic film and the spacerfilm.
 15. The medium of claim 1 wherein the second regions areion-irradiated regions.
 16. The disk of claim 10 wherein the secondregions are ion-irradiated regions.